Typically, a multiple-site trunked radio frequency (RF) communication system includes communication units, communication sites, communication resources, and a communication resource allocator, also known as a central controller, site controller, or zone controller, which may reside within base stations or repeaters located at the communication sites. Each site has a substantially distinct coverage area and is geographically located within the system. A number of communication resources are assigned to each site, where at least one of the communication resources is used as a control channel, while the remaining communication resources are used as voice and/or data channels, hereafter referred to simply as channels. The base stations or repeaters are used to generate the communication resources and are located at the site. The communication resources include, among other media, telephone connections, TDM time slots, spreading patterns, carrier frequencies, and/or frequency pairs. Some communication systems are wide-area communication systems and comprise many communication sites, which may be grouped into communication zones to more effectively allocate the communication resources among the communication units.
The communication units, also called subscribers or subscriber units, are often arranged in talkgroups and may be located anywhere within the system (in the coverage area of any site) when communicating within the system. When a communication unit from a talkgroup requests a group call, the communication unit transmits, via the control channel of the site in which it is located, an inbound signalling word (ISW) to the central controller or other controlling entity. A group call allows all members of the same talkgroup that are located within the system to communicate with each other. The ISW generally comprises the requesting communication unit's individual identification number, the requesting communication unit's talkgroup, and a request for a group call. The ISW may contain more or less information, depending on the system.
Upon receiving the ISW, the central controller determines if there is an available channel to allocate for each site in which a member of the talkgroup presently is registered. If a channel is available in each of these sites, the central controller will place the group call and transmits an outbound signalling word (OSW) to the requesting communication unit and each member of the talkgroup. If a channel is not available in each of these sites, the central controller will not place the group call and instead transmits an outbound signalling word (OSW) to the requesting communication unit. The central controller may try to convert this request into an active call at a later time when resources are available.
At times, when a mobile communication unit is active in a group or individual call, the communication unit travels out of the coverage area of its current site, causing the call to become degraded and even lost. The call may be handed off to another site for completion before much degradation or loss. Many different methods of call hand off exist. Typically, the communication unit tunes to a control channel and issues an ISW at a new site. If communication resources are available at the new site, a new resource is assigned, a channel grant is issued, and the communication unit continues the call on the new resource. If communication resources are presently assigned for the call at the new site, an OSW is issued to that effect, and the communication unit continues the call on the presently assigned resources. In general, call handoff can delay communications and utilises valuable bandwidth.
Some communication units have used a fixed rule or criterion for selecting a new site for a call handoff, such as a single signal quality threshold. A fixed rule, however, does not provide for adaptability to changing system conditions or any other dynamic criterion.
Accordingly, there is a need for a method of call handoff by a communication unit that is flexible and adaptable, yet does minimises delays and bandwidth utilisation.